Integrating circuits are well-known in the art and are used in various types of applications where conversion of light energy into electrical signals is required. Such applications for instance include image sensing in digital cameras or other imaging systems, motion sensing in optical pointing or scanning devices, signal detection in optical pickup devices such as CD-ROM or DVD-ROM drives, etc.
FIG. 1 schematically shows the known configuration of an integrating circuit where reference numeral 10 designates the photodetector PD which, when subjected to illumination, produces a current (or photo-generated current) IPD which is proportional to light intensity on photodetector 10. This current IPD is converted to a voltage signal VOUT by means of an amplifier circuit 30 and a capacitor 20 (commonly referred to as “integration capacitor”) of capacitance value CINT which is connected between an input 31 and an output 32 of amplifier 30. Amplifier 30 may include another input (not represented) connected to a reference potential such as ground.
FIG. 2 schematically shows a known configuration of photodetector 10 which consists of a photodiode comprising a well 11 of a first conductivity type (in this illustration an n-type well) diffused in a semiconductor substrate (or well) 12 of a second conductivity type (here a p-type semiconductor substrate). The well 11 forms the photosensitive region of the photodetector 10 and is coupled to the amplifier's input 31 as shown in FIG. 1 (typically through an n+ diffusion region in n-well 11 which is not represented in FIG. 2).
Integration capacitor 20 is typically formed as a distinct component requiring additional surface area on the chip. This integration capacitor 20 can typically be built as a stack of two layers of polysilicon or of metal and polysilicon layer with an intermediate insulating layer or as a capacitor-connected MOS device (for instance a MOS transistor having source and drain terminals connected together). All these solutions require area to implement.
In the above-mentioned applications, the optical sensing device typically comprises an array of a plurality of photodetector-integrator pairs. Layout space is usually at a premium and both the photodetector and the integration capacitor can take a lot of area. Tradeoffs must be made between the sizes of them. Typically the photodetector and integration capacitor are large and thus form a difficult constraint to any optimisation plan. One aim of the present invention is thus to find a solution to this problem, i.e. provide a solution which helps optimising the surface area of light conversion integrating circuits.